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Hi - I want to thank everyone for the input on the "did I buy the wrong furnace" thread - lots of good data, and the thread went in more directions than I anticipated.

Lost in the discussion however, was one question I was really really trying to get an answer to...

If I compare a single-stage FHA furnace, and a two-stage furnace of the same size....

1) Compared to the lowest fan speed of the single stage, roughly how much slower are the 'extra-low' fan speeds in the 2-stage (the speeds that are used when only one stage is on)? 20%? 50%? more?

2) When both stages are on in the two stage, will the 2-stage designed to move -more- air than the single stage?

This information (which I should have asked for before the install) will go a long way to help me gauge whether the recommendations I received to avoid a 2-stage for my unusual, archaic duct system were correct.

Hate to put too much heat in the basement, people are in it much of the time...

by the way - I'm a certified Moron (or CFM). I just went to Bryant's site. The CFM for Heating on my furnace is 1720CFM. The CFM for the closest 2-speed furnace of about the same size (actually 7% larger) is 1770CFM on high, and 1215CFM on low - which means the low-speed would be 30% lower than what i have now.

Now - what muddies the waters a bit is that my current furnace is a 4-speed, and they only list 1 CFM for heat (Which I think is the 2nd-to-lowest speed, and I'm already on the lowest, which could be like 1600 CFM). But, it boils the question down to - given my duct system as described at the top of this post, is 1215CFM too little in any case? I suspect it is....

Not sure what you mean by 'temp imbalance'. the old furnace (nominally the same output BTU) had longer cycles and had less 'velocity noise' (which I still think is -related- to cfm, no?) than the new one.

It's possible that my 45yr old, (112KBTU out) 80% efficient furnace was really less than 80% efficient, so that the new one, which really DOES put out 112KBTU, is over-sized, but I reserve judgement on that until we get some 0DegF weather.

I was thinking - it's possible that if I opened all the ducts all the way the 1215CFM might move enough air through the system so that the the temp at the furnace didn't go too high, and it would run longer cycles and be quiet. HOWEVER 8-}, when the 2nd stage cut in - whoosh - the noise would be annoying, and the system would be unbalanced (some rooms would cook waiting for others to get warm...).

Having an older stupid duct system sucks (sigh).

(btw - according to my somewhat accurate oven thermometer, the air temp right over the furnace is about 155DegF)